1. Field of the Invention
The present invention relates to a device that uses a linear variable differential transformer (LVDT) for measuring rotation of a shaft.
2. General Background and State of the Art
A linear variable differential transformer (LVDT) is a displacement transducer that produces an electrical signal proportional to the displacement of a moveable core (armature) within a cylindrical transformer. The transformer consists of a central, primary coil winding and two secondary coil windings on opposite ends of the primary winding. The coil windings are coaxial. The armature preferably is nickel-iron and is positioned within the coil assembly. The core provides a path for magnetic flux linking the primary coil to the secondary coils.
When the primary coil is energized with an alternating current, a cylindrical flux field is produced over the length of the armature. This flux field produces a voltage in each of the two secondary coils that varies as a function of the armature position. Armature movement moves the flux field into one secondary and out of the other causing an increase in the voltage induced in one secondary and a corresponding voltage decrease in the other. The secondary coils are normally connected in series with opposing phase. The net output of the LVDT is the difference between the two secondary voltages. When the armature is positioned symmetrically relative to the two secondary windings (the “null” position), the differential output is approximately zero, because the voltage of each secondary is equal but of opposite phase.
Subjecting an actuator to pressure or force can move an LVDT armature through a linkage. Thus, LVDTs are commonly used in pressure transducers. As pressure increases, the armature moves toward one secondary winding and away from the other. This yields a voltage difference that can be proportional to the pressure on the transducer. Consequently, this voltage output can measure pressure and position.
Nearly all LVDTs that are designed for aircraft or missile applications are wound on an insulated stainless steel spool, magnetically shielded and enclosed in a stainless steel housing using welded construction. The armature is normally made from a 50% nickel-iron alloy and brazed to a stainless steel extension. Secondary leads are usually shielded to minimize channel-to-channel crosstalk for multi-channel units and to shield components from RF energy.
The length and diameter of an LVDT must be sufficient to allow adequate winding space for achieving the desired electrical performance, support any pressure requirement and withstand the environmental shock, vibration and acceleration. Where physical size is limited, electrical performance must be flexible. Although the LVDT is basically a simple device, the operating characteristics and electrical parameters are complex and depend to a large extent on the physical limitations.
U.S. patent application Ser. No. 09/547,511, filed Apr. 12, 2000, which is assigned to the assignee of this application, discusses some of the parameters that designers consider when specifying the sizes of LVDT components. That discussion and the remainder of the application are incorporated by reference.
An LVDT's output voltage is proportional to the voltage applied to the primary. System accuracy depends on providing a constant input to the primary or compensating for variations of the input by using ratio techniques. The output can be taken as the differential voltage or, with a center tap, as two separate secondary voltages whose difference is a function of the displacement. If the sum of the secondary voltages is designed to be a specific ratio of the difference voltage, overall accuracy significantly improves.
Most LVDTs deal with a linear mechanical input in that a force acts on a probe that directly or indirectly carries the armature of the LVDT. The force moves the probe longitudinally along the axis of the probe. The device described in Ser. No. 09/547,511 is such a device. The output of some systems is rotational instead of linear. Consequently, there is a need for transducers that can measure the extent of a rotation. Rotary variable differential transducers (RVDT) having a circular geometry are known. They have been employed to measure the angular position of a rotary shaft. Known rotary variable differential transformers are more difficult and expensive to manufacture than LVDTs, however.
In some instances, tying the rotational output to a single LVDT produces acceptable results. However, many systems benefit with increased accuracy from multiple LVDTs.